1. Field of the Invention
The present invention relates to a communication method suitably applied to television broadcasting, for example, as well as to a transmission method, a transmission apparatus, a receiving method and a receiving apparatus.
2. Description of the Related Art
For carrying out a television broadcasting efficiently, what is called a digital television broadcasting has been put to practical use in which a video program conformed of a video signal, its associated audio signal and so on is converted to digital data and then transmitted. According to this digital television broadcasting, a number of video programs can be transmitted at the same time within a narrow transmission band, and a transmission band prepared for the television broadcasting can be used at high efficiency.
FIG. 1 is a diagram showing an example of a transmission arrangement used at a conventional transmitting station for transmitting a digital television broadcasting wave. Sources A, B, . . . , P, each being formed of an individual video program converted to respective digital data, are multiplexed in a multiplexing circuit 11 by a predetermined method. The signal thus multiplexed is encoded by an encoder 12 for transmission, and the output of the encoder 12 is phase-modulated by a QPSK modulation circuit 13 for transmission. Depending on a system configuration, the process up to the encoding by the encoder may be executed independently for each source and the output of the encoder may be multiplexed.
The signal modulated by the encoder 12 is supplied to an interleaver 14 which carries out an interleaving process to produce data over a predetermined section. The output of the interleaver 14 is applied to an inverse fast Fourier transform (IFFT) circuit 15 for producing a multicarrier signal by inverse fast Fourier transform. For example, a 128-point IFFT circuit 15 is used to arrange 64 carriers at a predetermined frequency interval and transmission data is modulated to be distributed over the respective carriers as a multicarrier signal.
The multicarrier signal output from the IFFT circuit 15 is supplied to a transmission windowing circuit 16 for multiplying a windowing data for transmission. The signal thus windowed is supplied to a digital/analog converter 17 and converted into an analog signal thereby. The output thus converted is supplied through a low-pass filter 18 to a frequency conversion circuit 19. This frequency conversion circuit 19 changes the frequency to a predetermined transmission frequency by multiplying the input with a signal output from an oscillator 20. The signal thus frequency-converted is supplied to a radio-frequency transmission circuit such as an amplifier 21 and so on to be subjected to a transmission process and then transmitted in a wireless fashion.
The transmission signal generated by the above process, as shown in FIG. 8, for example, is transmitted as a multicarrier signal in which a great number of carriers are arranged in one transmission band, and the data from the respective sources are distributed to the respective carriers within one transmission band and then transmitted. In this case, as the band width of one transmission band, if the number of sources to be transmitted at a time is many, a large number of frequency band widths are required.
Now, a configuration for receiving the broadcast wave transmitted with the configuration shown in FIG. 1 will be explained with reference to FIG. 2. An output of a receiving amplifier 31 connected to a receiving antenna or the like is supplied to a frequency conversion circuit 32, in which a signal in a predetermined transmission band (the frequency band converted by the frequency conversion circuit 19 at the transmitting side) is converted into an intermediate frequency signal based on the output of an oscillator 33, and the resulting intermediate frequency signal is supplied to an analog/digital converter 35 through a low-pass filter 34 thereby to produce digital data sampled at a predetermined sampling frequency.
The data thus sampled is multiplied by windowing data at a receiving windowing circuit 36 inverse to that used for the transmission. The data thus multiplied is supplied to an FFT (fast Fourier transform) circuit 37 where the multicarrier signal is restored into single-system data by the fast Fourier transform. For example, a multicarrier signal associated with 64 carriers is restored into single-system data using a 128-point FFT circuit 37. The data thus subjected to the fast Fourier transform is supplied to a deinterleaver 38 for restoring the data interleaved at the transmitting side into the original arrangement. Thereafter, the signal is supplied to a QPSK demodulation circuit 39 for demodulation thereby to produce a receiving symbol. The receiving symbol thus demodulated is decoded at a decoder 40 thereby to produce receiving data. This receiving data contains the data of all the sources multiplexed at the transmitting side. A desired source is extracted by a predetermined extraction circuit 41, and the data of the source thus extracted (the video signal and the audio signal) is supplied to a video equipment such as a television receiver or the like.
By performing the transmission and the receiving processes in this way, a plurality of video programs can be efficiently transmitted by the digital data in the form of a multicarrier signal.
In the case where a television broadcast wave is transmitted with the configuration described with reference to FIGS. 1 and 2, however, the burden on the circuit at the receiving side is inconveniently very great. Specifically, in the case of the conventional transmission with a multicarrier signal, all the video programs prepared are distributed uniformly to the respective carriers and then transmitted. In the receiving apparatus shown in FIG. 2, therefore, the circuits up to the decoder 40 are required to handle the signals in all the bands transmitted. As the low-pass filter 34, for example, a filter having a pass band to extract all of one transmission band shown in FIG. 3 is required. Also, the FFT circuit 37 is required to convert all the multiplexed carriers, and the deinterleaver 38 is required to carry out such a deinterleaving process to store all the multiplexed data and change the sequence thereof. Further, the demodulation circuit 39 is required to demodulate all the data multiplexed.
The data actually required at the receiving side, however, is only one arbitrary video program contained in the multiplexed data. It is therefore wasteful to receive and process all the data, and the circuit of the receiving system is unnecessarily enlarged in scale.